1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG: Arrayed Waveguide Grating) type optical multiplexer/demultiplexer applicable as a wavelength selection element to wavelength division multiplexing (WDM: Wavelength Division Multiplexing) transmission systems.
2. Related Background Art
The AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely applied to the wavelength selection elements in the WDM transmission systems, as wavelength filters enabling extraction or insertion of a specific wavelength by interference. The AWG circuits do not require so precise machining as required by diffraction gratings or so precise multilayer film formation as required by interference films, but they can be constructed by ordinary microprocessing such as lithography, etching, and so on. Therefore, the AWG circuits are expected to develop into dominant optical components in the future WDM transmission systems, also including the possibility of integration with other light waveguide elements.
Such AWG circuits have the structure in which an input waveguide, an input slab waveguide, channel waveguides (phased array) of mutually different lengths, an output slab waveguide, and output waveguides are integrated on a single substrate.
A variety of improvements have been proposed heretofore in the conventional AWG circuits and, for example, in order to decrease loss variation among signal channels (for flattening of passband), M. R. Amersfoort, et al., xe2x80x9cPassband broadening of integrated arrayed waveguide filters using multimode interference couplers,xe2x80x9d ELECTRONICS LETTERS 29th February, 1996, Vol. 32, No. 5 discloses the AWG circuit provided with a multimode interference coupler (MMI coupler: MultiMode Interference coupler, which will be referred to hereinafter as an MMI coupler) of the shape as illustrated in FIG. 12A, in the connection part between the input waveguide and the input slab waveguide (a first conventional example). Japanese Patent Application Laid-Open No. H09-297228 discloses the AWG circuit provided with a parabolic shape waveguide as illustrated in FIG. 13A, in the connection part between the input waveguide and the input slab waveguide (a second conventional example). Further, Japanese Patent No. 3039491 discloses the AWG circuit provided with a waveguide formed in the structure of combination of a tapered waveguide with a parabolic waveguide in the connection part between the input waveguide and the input slab waveguide (a third conventional example).
The inventor investigated the conventional AWG circuits of the structures as described above (the first to third conventional examples) and found out the following problems. Namely, the AWG circuit of the first conventional example is provided with the MMI coupler of the rectangular shape as illustrated in FIG. 12A, in the connection part between the input waveguide and the input slab waveguide. In this AWG circuit of the first conventional example, an electric field strength distribution of light having propagated through the input waveguide is badly disturbed during propagation in the MMI coupler, as illustrated in FIG. 12B, even if the MMI coupler is precisely processed. This disturbance of the electric field strength distribution is mainly caused by multiple reflection in the MMI coupler and much higher accuracy is required in processing for the width, length, etc. of the MMI coupler in order to achieve stable optical characteristics among the AWG circuits to be fabricated.
On the other hand, every AWG circuit of the above second conventional example is provided with the parabolic waveguide as illustrated in FIG. 13A. In this AWG circuit, the electric field strength distribution of light propagating in the parabolic waveguide is rarely disturbed, as illustrated in FIG. 13B, because the effect of multiple reflection is reduced in the parabolic waveguide. However, the electric field strength distribution of the propagating light becomes broadening according to the propagation in the parabolic waveguide (i.e., the peak-to-peak separation of the electric field strength distribution gradually increases). Since the parabolic waveguide has the small slope dy/dx (where the y-axis is taken along the longitudinal direction of the parabolic waveguide and the x-axis along the normal direction to the longitudinal direction in FIG. 12A) of the side faces even near the optical output end face thereof, variation can occur among the optical characteristics of the respective AWG circuits produced without attainment of sufficient processing accuracy or the like even in this second conventional example. This also applies to the AWG circuit of the third conventional example provided with the similar parabolic waveguide.
The present invention has been accomplished in order to solve the problems as described above and an object of the invention is to provide an optical multiplexer/demultiplexer having structure capable of relaxing the processing accuracy or the like required for realizing improvement in transmission wavelength characteristics, such as decrease in the loss variation among the signal channels or the like, i.e., structure capable of realizing higher manufacturing tolerance.
An optical multiplexer/demultiplexer according to the present invention comprises a substrate, at least one input waveguide provided on the substrate, a first slab waveguide, a plurality of channel waveguides, a second slab waveguide, and a plurality of output waveguides provided corresponding to respective signal channels, and is an AWG optical multiplexer/demultiplexer applicable as a wavelength selection element to the WDM transmission systems.
In the optical multiplexer/demultiplexer according to the present invention, each of the first and second slab waveguides has a predetermined slab length. The slab length is normally equivalent to a focal length of an optical input end functioning as a lens surface of each slab waveguide. The input waveguide is a waveguide for guiding signals of channel wavelengths set as signal channels at predetermined wavelength intervals, each to the first slab waveguide, and an optical output end thereof is connected to an optical input end face of the first slab waveguide. The channel waveguides are waveguides of mutually different lengths and are flatly arrayed on the substrate in a state in which optical input ends of the respective channel waveguides are connected to an optical output end face of the first slab waveguide so as to place the first slab waveguide between the input waveguide and the channel waveguides while optical output ends of the respective channel waveguides are connected to an optical input end face of the second slab waveguide so as to place the second slab waveguide between the channel waveguides and the output waveguides. Further, the above output waveguides are waveguides flatly arrayed on the substrate in a state in which optical input ends thereof are connected to an optical output end face of the second slab waveguide and are waveguides for individually taking out the signals of the channel waveguides set at the predetermined wavelength intervals.
Particularly, the optical multiplexer/demultiplexer according to the present invention comprises a waveguide provided between the input waveguide and the first slab waveguide, said waveguide being a free propagation area for coupling part of the fundamental mode of light having propagated through the input waveguide, to a higher order mode. This free propagation area is comprised of a first portion having side faces which extend along predetermined curves so as to increase width from an optical output end of the input waveguide toward an optical input end face of the first slab waveguide, and a second portion provided between the first portion and the first slab waveguide and having width larger than that of the input waveguide. Therefore, the first portion functions to broaden the electric field strength distribution of the light propagating from the optical output end of the input waveguide toward the optical input end face of the first slab waveguide by diffraction. On the other hand, the second portion is a waveguide having side faces nearly parallel to each other, which functions to limit the broadening of the electric field strength distribution of the light having passed through the first portion. When the free propagation area located between the input waveguide and the first slab waveguide is comprised of the first and second portions having the structures as described above, the tolerance of manufacturing errors necessary for attainment of desired optical characteristics, for example, for attainment of improvement in the transmission wavelength characteristics (e.g., reduction of the loss variation among the signal channels) becomes wider, so as to relax the required accuracy for microprocessing or the like.
The predetermined curves are preferably curves represented by exponential functions. Particularly, when the second portion is also processed so that its side faces extend along these curves, the free propagation area can be formed as a waveguide having continuous side faces. In this case, the side faces of the free propagation area go into almost parallel relation with distance from the optical input end face of the free propagation area, so that the input-waveguide side of the free propagation area functions as the first portion while the first-slab-waveguide side of the free propagation area as the second portion.
The above curves are given, for example, by the following equations. Namely, in an x-z coordinate system agreeing with a surface of the substrate, a reference line Xp(z) passing each of center points (xs, zs), (xe, ze) in an optical input end face and an optical output end face of the first portion being surfaces parallel to the x-axis, is given by the following equation:             Xp      ⁢              (        z        )              =                  x        s            +                                                  x              e                        -                          x              s                                                          z              e                        -                          z              s                                      ⁢                  (                      z            -                          z              s                                )                      ,
the width w(z) of the first portion parallel to the x-axis is given by the following equation:             w      ⁢              (        z        )              =                  w        s            +              A        ⁡                  (                                    exp              ⁡                              (                                                      -                    α                                    ⁢                                                            z                      -                                              z                        s                                                                                                            z                        e                                            -                                              z                        s                                                                                            )                                      -            1                    )                      ,      xe2x80x83    ⁢      A    =                            w          e                -                  w          s                                      exp          ⁡                      (                          -              α                        )                          -        1            
ws: the width of the optical input end face of the first portion,
we: the width of the optical output end face of the first portion,
xcex1: an exponential coefficient,
and under the above conditions the curves are given by the following equations (see FIG. 4):                     x        l            ⁢              (        z        )              =                            x          p                ⁢                  (          z          )                    -                        w          ⁢                      (            z            )                          2              ,      xe2x80x83    ⁢                    x        n            ⁢              (        z        )              =                            x          p                ⁢                  (          z          )                    +                                    w            ⁢                          (              z              )                                2                .            
In this case, where the free propagation area is integrally processed so that the side faces of the first and second portions all extend along the curves defined as described above, i.e., where we is the width of the optical output end face of the free propagation area, a portion with the width in the range of (1xe2x88x92xcex94)we to we (a portion in which absolute values of slopes dz/dx of the above curves are not less than we/xcex0 where the normal direction to the optical input end face of the first slab waveguide agrees with the z-axis) functions as the second portion. Here xcex94 represents an infinitesimal change rate given by scalar quantity xcex0/we, where xcex0 is the center channel wavelength.
Further, it is preferable in the optical multiplexer/demultiplexer of the present invention that a relative refractive-index difference between cores corresponding to the waveguides (including the above-stated input waveguide, free propagation area, first and second slab waveguides, channel waveguides, output waveguides, and so on) provided on the substrate and a cladding provided on the substrate so as to cover the cores be not less than 1%. In addition, the input waveguide, the channel waveguides, and the output waveguides all preferably have the width (core width) of not more than 5.5 xcexcm. The reason is that it increases the optical confinement effect of light into the cores (corresponding to the respective waveguides) and more relaxes the required accuracy for microprocessing etc. of the free propagation area and the like. In addition, the improvement in the optical confinement effect permits the core width to be decreased more, and this makes it possible to provide more channel waveguides on the substrate. The optical multiplexer/demultiplexer can be obtained with excellent optical characteristics, because the theoretically minimum spacing between adjacent cores, which is a constraint on the production process, so called xe2x80x9cembedding limit,xe2x80x9d can be made smaller.
The optical multiplexer/demultiplexer according to the present invention may further comprise an MMI coupler provided between the optical output end of the input waveguide and an optical input end of the first portion in the free propagation area. This structure permits the required accuracy for microprocessing or the like to be relaxed more when compared with the conventional, optical multiplexer/demultiplexers and also permits further improvement in the transmission wavelength characteristics of the optical multiplexer/demultiplexer.
In either of the structure wherein the free propagation area is provided between the input waveguide and the first slab waveguide and the structure wherein the MMI coupler is provided in addition to the free propagation area, it is preferable that the optical output end of the input waveguide be spaced 2.0 or more xcexcm apart from the optical input end face of the first slab waveguide, i.e., that the total length of either the free propagation area or the free propagation area plus the MMI coupler be not less than 2.0 xcexcm. This can restrain work bluntness easy to occur in the connection part between the free propagation area and the first slab waveguide (bluntness of the shape both in the vicinity of the optical output end face of the free propagation area and in the vicinity of the optical input end face of the first slab waveguide forming the connection part) and also permit production of the free propagation area with good embeddability of cladding into gaps between the cores by making use of the CVD (Chemical Vapor Deposition) technology and FHD (Flame Hydrolysis Deposition) technology.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.